Wind power is considered one of the cleanest, most environmentally friendly energy sources presently available, and wind turbines have gained increased attention in this regard. A modern wind turbine typically includes a tower, generator, gearbox, nacelle, and one or more rotor blades. The rotor blades capture kinetic energy of wind using known airfoil principles. For example, rotor blades typically have the cross-sectional profile of an airfoil such that, during operation, air flows over the blade producing a pressure difference between the sides. Consequently, a lift force, which is directed from a pressure side towards a suction side, acts on the blade. The lift force generates torque on the main rotor shaft, which is geared to a generator for producing electricity.
During operation, wind impacts the rotor blades of the wind turbine and the blades transform wind energy into a mechanical rotational torque that rotatably drives a low-speed shaft. The low-speed shaft is configured to drive the gearbox that subsequently steps up the low rotational speed of the low-speed shaft to drive a high-speed shaft at an increased rotational speed. The high-speed shaft is generally rotatably coupled to a generator so as to rotatably drive a generator rotor. As such, a rotating magnetic field may be induced by the generator rotor and a voltage may be induced within a generator stator that is magnetically coupled to the generator rotor. In certain configurations, the associated electrical power can be transmitted to a turbine transformer that is typically connected to a power grid via a grid breaker. Thus, the turbine transformer steps up the voltage amplitude of the electrical power such that the transformed electrical power may be further transmitted to the power grid.
In many wind turbines, the generator rotor may be electrically coupled to a bi-directional power converter that includes a rotor side converter joined to a line side converter via a regulated DC link. More specifically, some wind turbines, such as wind-driven doubly-fed induction generator (DFIG) systems or full power conversion systems, may include a power converter with an AC-DC-AC topology. In such system, the generator stator is separately connected to the power grid via a main transformer.
The power converter usually includes several switching devices such as insulated gate bipolar transistors (IGBTs), integrated gate commutated thyristors (IGCTs or GCTs), diodes, and/or metal oxide semiconductor field effect transistors (MOSFETs) that are switched at certain frequencies to generate the desired converter output voltage and frequency. The converter output voltage is then provided to various loads such as motors, power grids, resistive loads, etc.
IGBTs and other power semiconductors have a failure mode based on cosmic rays. High-energy particles from outer space travel into the earth's atmosphere and collide with an atomic nucleus in the outer atmosphere, thereby creating a multitude of secondary particles which carry away the energy of the primary particle. These particles can randomly hit the IGBTs and other power semiconductors with a probability based on the size of the device as well as altitude. If the IGBT is supporting voltage when hit by one of these particles (hence the semiconductor junction is acting in the blocking region), the particle(s) can create electron-hole pairs which carry current in very small portions of the IGBT. Depending on the voltage across the IGBT and the energy level of the particle(s), the IGBT may be permanently damaged.
Thus, the present disclosure is directed to systems and methods for operating a power converter to improve IGBT reliability by addressing the aforementioned issues.